This invention relates generally to systems for reducing the discharge of pollutants from fuel storage tanks, and in particular to vapor recovery systems for fuel storage tanks.
When fuel is added to a fuel reservoir, such as the gasoline tank of an automobile, from a conventional gas dispenser, such as the dispensing nozzle of a gasoline dispenser, gasoline vapor is displaced from the gasoline tank. If the vapor is not collected in some manner, it will be released into the atmosphere. Due to the large number of automobile refuelings, such releases of fuel vapor constitute a significant hazard to the environment, particularly in heavily populated areas. Releases of these vapors, which are composed of volatile organic compounds (VOCs) such as hydrocarbons, are presently the subject of significant and increasing federal and state regulation.
In order to guard against the release of VOCs to the environment, several vapor collection systems have been designed and implemented to collect the vapors displaced from automobile gasoline tanks during refueling. All of these systems collect the vapors during refueling and discharge them into the underground fuel storage tank. These systems have proven to be very capable of transporting the vast majority of the vapor from the filler pipe of the automobile to the underground fuel storage tank. However, in some cases, the act of pumping the vapor can lead to pressurization of the underground fuel storage tank and the associated piping. In addition, other factors, such as temperature changes, can lead to pressurization.
Another manner in which fuel storage tanks can become pressurized is during the filling of the fuel storage tank by a fuel tanker truck. In particular, conventional fuel tanker trucks include vapor recovery systems for collecting fuel vapors displaced from the fuel storage tank during the filling process. However, the vapor recovery systems of conventional fuel tanker trucks are typically not capable of collecting all of the displaced vapor, or the vapor recovery systems may not be operated properly by the operator. As a result, the fuel storage tank can become pressurized by the displaced vapors created during the filling process.
The underground fuel storage tanks and piping include an area above the liquid known as the ullage, in which air and fuel vapors reside. The pressurized air and fuel vapors within the ullage will have a tendency to leak out of any hole in the tank or piping of the system, or they can be released from the ullage through a pressure relief valve, thus allowing the release of the polluting VOC vapor to the atmosphere. Thus, controlling the pressure of the ullage is an important component of minimizing the fugitive release of VOC vapors.
Conventional systems for controlling the pressure of the ullage have relied upon vapor recovery systems that pump vapors from the ullage, adsorb the VOC vapors, and vent the non-polluting VOC-free air into the atmosphere. The VOCs are typically adsorbed using a plurality of VOC adsorbent canisters. During the vapor recovery process, a first canister adsorbs VOCs. When the first VOC adsorbent canister becomes saturated, a second VOC adsorbent canister is used to adsorb VOCs, and the first VOC adsorbent canister is regenerated by sweeping VOC-free air through the first VOC adsorbent canister. This cycle of adsorption and regeneration alternates between the canisters until the tank ullage pressure is reduced to a threshold level. Such conventional vapor recovery systems for fuel storage tanks are complex and expensive to build and maintain due to the required valving and controls for switching the canisters during the adsorption and regeneration cycles. Furthermore, the regeneration of the VOC adsorbent canisters by sweeping VOC-free air through the saturated canisters introduces a large volume of air into the ullage thereby further pressurizing the ullage. Finally, the conventional vapor recovery systems do not eliminate fugitive emissions of VOCs from fuel storage tanks during dispensing of the fuel, periods of non-use, and intervals of time when fuel is being delivered to the fuel storage tank.
The present invention is directed to overcoming one or more of the limitations of existing approaches to recovering vapors in fuel storage tanks.
According to an embodiment of the present invention, a vapor recovery system for a fuel storage tank having an ullage including VOC vapors and non-VOC vapors is provided that includes an ullage pressure sensor coupled to the ullage for sensing the operating pressure of the ullage, a vapor pump coupled to the ullage for pumping vapors from the ullage, a vacuum pump coupled to the ullage for exhausting vapors into the ullage, a first canister containing a VOC adsorbent material for adsorbing VOC-vapors, a second canister containing a VOC adsorbent material for adsorbing VOC-vapors, an atmospheric vent coupled to the first and second canisters for conveying vapors exhausted by the canisters to the atmosphere, a first recovery valve coupled between the vapor pump and the first canister for controlling the flow of vapors from the vapor pump to the first canister, a second recovery valve coupled between the vapor pump and the second canister for controlling the flow of vapors from the vapor pump to the second canister, a first regeneration valve coupled between the vacuum pump and the first canister for controlling the exhaustion of vapors from the first canister by the vacuum pump, a second regeneration valve coupled between the vacuum pump and the second canister for controlling the exhaustion of vapors from the second canister by the vacuum pump, and a controller coupled to the pressure sensor, the vapor pump, the vacuum pump, the recovery valves, and the regeneration valves. If the sensed operating pressure of the ullage exceeds a first set point, the controller is adapted to operate the vapor pump to pump vapors out of the ullage, and operate the first recovery valve to permit vapors pumped by the vapor pump to flow through the first canister. The VOC-vapors are adsorbed within the first canister and substantially all of the non-VOC vapors are exhausted from the first canister to the atmosphere through the atmospheric vent.
According to another embodiment of the invention, a VOC adsorbent canister for use in a vapor recovery system for a fuel storage tank having an ullage including VOC-vapors and non-VOC vapors is provided that includes a housing containing VOC adsorbent materials and a strain gauge coupled to the exterior surface of the housing for measuring the weight of the housing.
The present embodiments of the invention provide a number of advantages. For example, the use of a pair of VOC adsorbent canisters that are alternately used to adsorb VOC vapors and regenerated provides substantially constant adsorption of VOC vapors from the ullage. Moreover, the use of a vacuum pump to exhaust VOC vapors from the saturated VOC adsorbent canisters and thereby regenerate the canisters minimizes the amount of non-VOC vapor that is reintroduced into the ullage during the regeneration process. In this manner, the operating pressure of the ullage is minimally increased during the regeneration process. Furthermore, the use of a pair of VOC adsorbent canisters to recover VOC vapors during normal operating conditions by alternating between vapor recovery and regeneration provides an efficient and cost effective system for VOC vapor recovery. In addition, the additional parallel use of a third VOC adsorbent canister having increased VOC adsorbing capacity permits the system to effectively and efficiently handle increased operating pressures within the ullage. Also, the additional use of heaters for regenerating the VOC adsorbent materials within the VOC adsorbent canisters increases the rate at which the canisters may be regenerated. Finally, sensing the level of saturation of the VOC adsorbent canisters by weighing the canisters and/or monitoring the pressure of the VOC vapors provides a reliable method of monitoring the saturation level of the VOC adsorbent canisters.